Anyone with a late model Ford car or truck from the early 80's till probably now has no doubt experienced this problem with the plastic radiator tanks.
The petcock valve screws into the tank and seals on a rubber gasket. The first or second time that you unscrew the valve to either replace the coolant or service the engine, the valve will not seal properly again. The problem is that the threads on both the valve and the radiator tank strip easily and you now have a leak. You could buy a new radiator, but I have found an easier, cheaper and reliable way to permanently fix this problem.
Here is the valve in it's native state:
Once the valve is un-screwed, the threads are usually stripped and the valve will no longer seal:
Here you can see the hole that the water drains through at the top of the photo:
I use an expanding rubber plug to seal the main hole in the radiator. A plug that will fit a 1"-1 1/8" hole works perfect. When you turn the nut on the plug, the rubber expands and seals the hole:
The plug is placed in the radiator and tightened until snug:
A short piece of 3/8" hose, a 3/8" bolt and a couple of clamps will seal the drain hole:
The hose is placed on the nipple, and everything is tightened down. The next time you need to drain the coolant, you can remove the rubber hose to drain: